This disclosure relates to a method of reducing shadows when thick septa collimators are used for X- and gamma-ray imaging. In particular, it relates to a method of reducing shadows (obtained when thick septa collimators are used for imaging) via synthetic collywobbling.
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
Single photon imaging, either planar or SPECT, relies on the use of a collimator placed between the source and a scintillation crystal or solid state detector, to allow only gamma rays aligned with the holes of the collimator to pass through to the detector, thus constraining the line on which the gamma emission is able to occur.
Two principal types of multi-hole collimators have been used in nuclear medical imaging. One includes parallel-hole collimators while the other includes focusing collimators such as fan collimators, cone collimators, variable-focus collimators, and the like. The predominant type of collimator is the parallel-hole collimator. Both types of collimator contain thousands of holes formed into a very dense material such as lead. The holes of an ideal parallel-hole collimator all point perpendicular to the collimator surface and accept only photons traveling in that direction. It produces a planar image of the same size as the source object. The hole directions of an ideal focusing collimator are inclined relative to the collimator surface normal in a regular, mathematically well-defined manner; e.g., the holes of an ideal fan beam collimator all point to a line; the holes of an ideal cone beam collimator focus to a point. Focusing collimators magnify or minify the image depending on whether the holes converge or diverge.
Tomographic reconstruction requires accurate knowledge of these hole directions in order to infer the line of response from which the acquired projection data emanated. However, problems in construction and manufacture of real collimators can cause their hole directions to differ from the ideal. This degrades the quality of tomographic images because the projection and backprojection processes involved in tomographic reconstruction take place along distorted lines of response. As a remedy to this problem, nuclear vector maps are used to measure the actual direction of the holes so that tomographic reconstruction can take place along the true, rather than idealized, lines of response of the collimator. This improves the quality and accuracy of the resulting tomographic images.
Nuclear vector maps are measured by scanning a line source across the collimator field of view and measuring the location of the line at each scan point. The line location is defined by the center of the transverse profile through the line at each image location along the line. This location is then compared to a reference (calibrated) position of the line center. The hole angle is deduced from this geometry. This process is performed in two orthogonal dimension; e.g., the X, Y-dimensions of the imaging detector corresponding to scans using vertically and horizontally oriented line sources.
The walls surrounding and defining the collimator holes (septa) are designed to be sufficiently thick to absorb photons not traveling in the desired direction. For low energy isotopes such as Tc99 the septa are thin and produce no visible effects in the line images. At higher energies, however, the septal thickness must be increased to absorb the more penetrating photons. As the septal thickness increases, septal shadows are produced which distort images and produce artifacts. Septal artifacts distort the line images and their profiles.
These artifacts prevent an accurate determination of hole-direction angles, which in turn prevent obtaining accurate vector maps. It is therefore desirable to devise a method to reduce septal shadows so that hole orientation angles can be accurately computed if desired. Knowing hole orientation angles properly facilitates an accurate determination of vector maps and hence of the accuracy of tomographic imaging.